Improvements in the perceived sharpness of a displayed video image produced from a video signal, are typically accomplished with edge transient sharpness enhancement methods that employ an enhancement signal which is added to the video signal to steepen edge transients contained therein. Utilizing such an enhancement signal steepens the rise in a transition from a dark image region to a light image region, or alternatively, steepens the fall in a transition from a light image region to a dark image region.
Some prior art edge transient sharpness enhancement methods employ linear techniques which enhance frequencies that are already within the video image by using a linear high frequency boosting filter. Other prior art edge transient sharpness enhancement methods employ highly non-linear techniques that attempt to steepen the edge transients without introducing so-called under and overshoots which are introduced by the linear techniques. As such, truly new high frequencies are introduced into the displayed video image.
The linear techniques are used in some prior art edge transient sharpness enhancement methods to symmetrically modify video signal edge transients on both sides of the edge transient center by steepening both sides of the edge transient substantially equally. Other prior art edge transient sharpness enhancement methods use the non-linear technique to steepen the sides of the edge transient, but all in a symmetrical manner.
FIG. 1 is an enhanced edge transient generated by an exemplary prior art edge transient enhancement method that localizes the edge center of the edge transient of a video signal and steepens the edge transient on both sides of the edge center symmetrically. Solid line 10 represents an edge transient of a video signal having an edge center 12, a first edge side portion 14 and a second edge side portion 16. Solid line 18 represents an edge enhancement signal or edge change signal. The edge change signal 18 is generally symmetrical around its zero crossing, the position of which is located at the edge center 12 of the edge transient 10. The edge change signal 18 is added to the original video signal 10 to arrive at a video signal with an enhanced edge transient represented by dashed line 20. The enhanced edge transient 20 includes an edge center 22 which is substantially identical to edge center 12 of the original edge transient video signal 10, a first enhanced or steepened edge side portion 24 and a second enhanced or steepened edge side portion 26.
There are problems associated with these prior art edge transient enhancement methods. Thin (white) lines in the displayed video image appear wider than the same lines in the original video image, and if the enhancement is performed robustly, the resulting video image may appear very unnatural.
Accordingly, improvements in edge transient enhancement is needed to overcome the problems associated with prior art edge transient enhancement methods.